1. Field of the Invention
The present invention generally relates to a printer such as an electrophotographic printer in which a planetary gear for driving a printing mechanism is switched between two positions depending on the direction of rotation thereof. The present invention also relates to a method of controlling such a printer.
2. Description of the Related Art
FIG. 10 illustrates a conventional printer.
Referring to FIG. 10, a charging roller 12 rotates in contact with a photoconductive drum 11. The charging roller 12 receives a negative voltage from a charging power supply 10 and charges the surface of the photoconductive drum 11 to a negative potential of about xe2x88x92800 V. White circles on the photoconductive drum 11 indicate negative charges. A light source 13 in the form of, for example, an LED head illuminates the charged surface of the photoconductive drum 11 to form an electrostatic latent image on the surface. A developing unit 14 has a roller unit 15 consisting of a plurality of rollers. The developing unit 14 supplies negatively charged toner to the electrostatic latent image formed on the photoconductive drum 11 to develop the electrostatic latent image into a toner image. Black circles indicate toner particles. A transfer roller 19 rotates in contact with the photoconductive drum 11 and receives a positive voltage from a transfer power supply 18, thereby transferring the toner image formed on the photoconductive drum 11 to print paper 16. A cleaning unit 17 removes residual toner that failed to be transferred to the print paper 16 and was left on the photoconductive drum 11 after the transfer operation. A fixing unit 23 includes a heat roller 23a and a pressure roller 23b, so that when the print paper is pulled in between the heat roller 23a and pressure roller 23b, the toner image on the print paper 16 is fused. The heat roller 23a and the pressure roller 23b discharge the print paper 16 to the outside of the printer.
When the toner image is being formed on the photoconductive drum 11, the print paper 16 is fed from a paper tray 20. A feed roller 21 feeds the top page of a stack of print paper held in the paper tray 20 to the transfer point between the photoconductive drum 11 and the transfer roller 19. The pages of print paper are fed on a sheet-by-sheet basis. The print paper passes through a transport path in which a pair of registry rollers 22a and 22b is disposed.
The rollers of associated mechanisms are driven by the same motor via a plurality of gears. While some of these rollers may be driven to rotate and stop at the same timing, others may have to be controlled independently.
Immediately before a printing operation, a warm-up operation is performed in order to ensure stable printing operation. During the warm-up operation, the print paper should not be advanced and therefore if the print paper is to be fed from the paper tray 20, the feed roller 21 is prevented from rotating during the warm-up operation. Thus, the feed roller 21 must be controlled independently of the other rollers.
When the print paper is fed from the paper tray 20, the registry rollers 22a and 22b may be rotated and stopped at the same timing as the other rollers. However, when the user feeds the print paper 16 manually from the front side of the printer, the controller is unable to know the timing at which the print paper is actually fed. Thus, it is required that a paper sensor 24 accurately detects the position of the print paper so that the registry rollers 22a and 22b are driven into rotation at a proper timing independent of the other rollers. The print paper is accurately fed in this manner so that the printing is initiated at a specified location on the print paper.
As described above, if the rollers are necessary to be driven independently, they are usually driven by separate motors. However, small size, low price printers should be equipped with a minimum number of motors. Therefore, a desirable printing mechanism uses a planetary gear mechanism.
FIG. 11 illustrates a gear train used in a small printer and driven by a single motor, showing the meshing engagement among the gears during the warm-up operation.
The gear train is featured by a planetary gear 25g. The planetary gear 25g is in mesh with a gear 27g which in turn is in mesh with a gear 26g of a motor 26. The planetary gear 25g rotates in the same direction as the motor 26. The planetary gear 25g is movable about the gear 27g so that the position of the planetary gear 25g may be switched between two positions depending on the rotational direction of the planetary gear 25g. 
The operations of the gears during the warm-up operation will be described with respect to FIG. 11.
During the warm-up operation, the motor 26 drives the motor gear 26g in rotation in a direction shown by arrow A1. Thus, the planetary gear 25g rotates in a direction shown by arrow A2 so that the position of the planetary gear 25g is switched in a direction shown by arrow A3. The planetary gear 25g drives an idle gear 28g in rotation. Then, the idle gear 28g drives a triple gear 29g, which in turn drives a transfer roller gear 19g and a fixing roller gear 30g. The transfer roller gear 19g is mounted to the transfer roller 19 and drives the transfer roller 19 in rotation. The transfer roller gear 19g also drives the photoconductive drum 11, not shown, in rotation. The photoconductive drum 11 has another gear that drives other associated rollers such as the charging roller 12 and developing roller 15. A gear 30g drives rollers 23a and 23b of the fixing unit 23 in rotation.
In FIG. 11, the gear 31g is not in mesh with the planetary gear 25g, so that no drive force is transmitted to the feed roller gear 21g, gear 32g, and registry roller gear 22g. Thus, the print paper is not fed.
FIG. 12 illustrates the gear train during the printing operation. During the printing operation, the motor 26 rotates in a reverse direction, driving the motor gear 26g to rotate in a direction shown by arrow B1. Thus, the planetary gear 25g rotates in a direction shown by arrow B2, the position of the planetary gear 25g being switched in a direction shown by arrow B3. Then, the planetary gear 25g is brought into meshing engagement with the triple gear 29g and gear 31g, thereby driving the triple gear 29g and gear 31g in rotation.
The gear 31g drives the feed roller gear 21g. The feed roller gear 21g is operatively connected to the feed roller 21 through a clutch, not shown, so that the feed roller gear 21g drives the clutch to engage and disengage in accordance with a signal from a printer controller. The rotation of the feed roller gear 21g is transmitted through the gear 32g to the registry roller gear 22g so that the registry roller 22g causes the print paper to advance.
As mentioned above, the motor gear 26g is rotated in the direction shown by arrow B1 during the printing operation, rollers associated with the printing operation, rollers associated with fixing operation, and registry rollers are simultaneously rotated. The clutch is controlled by the signal from the printer controller, thereby causing the feed roller 21g to rotate and stop as required.
As described above, the conventional printer is designed to reverse the direction of rotation of the motor 26 depending on whether the print paper 16 should be fed and should not be fed. Then, the planetary gear 25g was used to control the transmission of the drive force of the motor 26.
In the manual feed mode, the user inserts the print paper from the front side of the printer. However, the printer controller does not know the timing at which the user inserts the print paper. Therefore, the warm-up operation cannot be performed immediately before the transport of the print paper. In other words, when the print paper is manually fed, the transport of the print paper should be halted as soon as the leading end of the print paper has been positioned just past the registry rollers 22a and 22b, and then the warm-up operation is performed before entering the printing operation.
As described above, with the printer of FIG. 11, when the print paper is manually fed, the registry rollers 22a and 22b are used to set the print paper at a predetermined position. The registry rollers 22a and 22b are rotated only when the motor 26 rotates in the direction shown by arrow B1.
Thus, when the print paper is inserted into the manual feed tray, not shown, the print controller receives a detection signal from the manual feed sensor 24 and causes the motor 26 to rotate in the direction of the arrow B1 as shown in FIG. 12. The print paper placed on the manual feed tray is pulled in between the registry rollers 22a and 22b. Before the leading end of the print paper reaches the transfer point (contact area) defined between the photoconductive drum 11 and the transfer roller 19, the motor 26 is stopped so that the print paper is held where it is. Then, as shown in FIG. 11, the motor 26 is rotated in the direction shown by arrow A1, thereby performing the warm-up operation just before the printing. Since the motor 26 rotates in the reverse direction, the planetary gear 25g is caused to move in the direction shown by arrow A3, so that the registry rollers 22a and 22b receive no drive force and therefore the print paper remains held in the transport path.
FIGS. 13 and 14 are timing charts, illustrating timings at which a high voltage is applied to the photoconductive drum 11 when the motor 26 and photoconductive drum 11 reach specific rotational speeds.
If the motor 26 is to begin to rotate in the same direction in which the motor 26 rotated just before the motor 26 stopped, then the planetary gear 25g remains at the same position. In other words, the planetary gear 25g begins to rotate from where it stopped. Thus, as shown in FIG. 13, the planetary gear 25g causes the gear in mesh with it to rotate simultaneously with the motor 26 begins to rotate.
However, as shown in FIG. 14, if the motor 26 is to rotate in a direction opposite to the direction in which the motor 26 was rotating before the motor 26 stopped, a time should be allowed for the planetary gear 25g to be switched from one position to the other. That is, it takes a length of time for the planetary gear 25g to properly mesh with another gear (e.g., from the gear 31g of FIG. 12 to the idle gear 28g of FIG. 11 during the warm-up operation). Thus, the photoconductive drum 11 and rollers start rotating a short time after the motor 26 has started rotating. Therefore, if a high voltage is applied to the photoconductive drum 11 at the same time that the motor 26 starts rotating, the photoconductive drum 11 receives the high voltage while it is still stationary. As a result, the photoconductive drum 11 may be damaged.
Moreover, when the planetary gear 25g has been brought into meshing engagement with associated gears, the motor 26 is still being accelerated or may have reached a high speed. Thus, the planetary gear 25g is also rotating at high speed. The planetary gear 25g rotating at high speed is suddenly brought into meshing engagement with the stationary mating gear, large loads being suddenly exerted on the planetary gear 25g and the stationary mating gear so that the motor 26 and gears may be subjected to mechanical damages.
If the surface of the photoconductive drum 11 is damaged such that the surface is not properly charged to about xe2x88x92800 V but to, for example, nearly 0 V, the toner is deposited thereto even though no image is actually formed in accordance with print data. This causes deteriorated print quality. Toner deposited on the photoconductive drum 11 forms black lines on the print paper, resulting in poor print quality.
Moreover, if the charging roller 12 and other rollers start rotating with no voltage applied to the charging roller 12, the toner between the charging roller 12 and the photoconductive drum 11 may migrate to the photoconductive drum 11, resulting in lateral lines on the printed image. Such a phenomenon also occurs between the developing roller 15 and the photoconductive drum 11 and between the transfer roller 19 and the photoconductive drum 11.
When a printing is performed in the manual feed mode, the warm-up operation is performed with the print paper not advanced. The printing is then started after the rotational direction of the motor 26 is switched. Thus, if the photoconductive drum 11 is contaminated with toner, the contamination causes soiling of print paper in most cases.
The present invention was made in view of the aforementioned problems.
An object of the invention is to provide a printer where when a drive force is transmitted through a planetary gear to associated gears in accordance with the rotational direction of a motor, voltages are applied at controlled timings and the motor rotates at a controlled speed.
A method of controlling is used to controllably drive a printing mechanism by using a planetary gear that is selectively positioned depending on a direction of rotation thereof. The method includes the steps of:
storing a first position of the planetary gear when the planetary gear is not rotating;
determining a second position to which the planetary gear should be positioned when the planetary gear starts rotating after stoppage, the second position being determined depending on the direction of rotation in which the planetary gear starts rotating;
changing a timing at which a high voltage is applied to an associated section for a printing operation, the timing being determined in accordance with the second position.
A printer has a printing mechanism and a drive mechanism that drives the printing mechanism. The printing mechanism includes a charging unit, an exposing unit, a photoconductive drum, a developing unit, and a transfer unit. The drive mechanism includes a planetary gear that is selectively positioned depending on a direction of rotation thereon. The printer comprises a memory and a controller. The memory stores a first position of the planetary gear at which the planetary gear stopped rotating. The controller determines a second position to which the planetary gear should be positioned when the planetary gear starts rotating after stoppage. The second position is determined depending on the direction of rotation in which the planetary gear starts rotating. The controller controls a timing at which a voltage is applied to the printing mechanism, the timing being determined in accordance with the first position and the second position. The controller applies the voltage to the printing mechanism at a first timing if the second position is the same as the first position, and at a second timing if the second position is different. from the first position.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.